November 18, 2005  

Polarized Sources Pioneer Prescott Retires

(Photo by Diana Rogers)

(Courtesy of SLAC Archives)

Prescott now (shown left) and from the 1975 SLAC Phonebook.

By Heather Rock Woods

Charles Prescott (EA) is retiring on December 1, after 34 years at SLAC leading the Lab and the world in developing and using polarized sources of electrons in high energy physics experiments.

Prescott is probably most known for developing the first high-intensity polarized source and heading the groundbreaking E-122 experiment that solidified the then-disputable electroweak theory, the foundation of today’s wildly successful Standard Model.

He also gains recognition each summer as one of the four directors of the SLAC Summer Institute. He will complete his tenth year as director next summer before handing the reins over to someone new.

“SSI is hard work and it’s rewarding,” Prescott said. “We get lots of positive feedback from lecturers and students.”

In true SLAC fashion, the ‘retired’ Prescott will still work half time (on ‘recall to active duty’). Relinquishing the administrative duties of Group A Leader (coinciding with the group’s dissolution), he will continue to do R&D for the neutrino experiment EXO—a career anomaly because the project uses no polarized source and no accelerator.

From Bubble Chambers to Polarized Beams

Prescott joined SLAC in 1971 as a research associate first working on the BC-42 bubble chamber experiment in Group A. He soon turned to polarization. In the 1970’s, experimental physicists wanted to probe for quarks—not yet a solved mystery—using deep inelastic scattering. In deep inelastic scattering, electrons scatter off protons, interacting with the quarks inside, at energies strong enough to break up the proton.

“Weinberg and Salam had a theory that unified the weak and electromagnetic forces,” Prescott said. “The theory predicted that in electromagnetic interactions, you should see a bit of the weak force.” Prescott proposed an experiment to test the theory of electroweak interactions by finding parity nonconservation— violation of mirror symmetry—in inelastic electron scattering.

To make the measurements, physicists needed a source of polarized electrons. In a polarized particle beam, the majority of the particles are aligned to spin in the same direction, like a clockwise spiral on a football as it speeds toward a receiver. Physicists expected that electrons polarized to be forward-spinning would interact with quarks at a slightly different rate than reverse-spinning electrons (a parity violating effect).

Because there was no way to polarize electrons into an intense enough beam, Prescott, then a permanent staff physicist, started work in 1974 with Roger Miller (ARDA), Ed Garwin and Charlie Sinclair (both PEL) to develop the first high-intensity polarized source.

They put their source to brilliant use in SLAC experiment E-122 in 1978. It was the first high-intensity polarized source on an accelerator and also the first use of gallium arsenide (a semiconductor material) for an accelerator. When struck by intense laser light, gallium arsenide emits a large number of polarized electrons. Most electron accelerator labs now use the material in their polarized sources.

The weak force has a different effect depending on whether you look for it with a forward-spinning electron (that you scatter off a proton) or a reverse-spinning electron. The experiment famously found this small asymmetry (10-4), thus proving that the weak force is involved in electron-quark interactions.

E-122’s success provided the cornerstone for acceptance of the electroweak theory, which at the time had competitors. A year later, in 1979, Steven Weinberg, Abdus Salam and Sheldon Glashow won the Nobel Prize for this theory. Prescott won the 1988 Panofsky Prize for his experimental work.

Sinclair working on the first high-intensity polarized source in the mid-1970's.

(Photo by Joe Faust)

Fixed Target and Colliding Beam Experiments

After E-122, Prescott became an associate professor and contributed to proposals and R&D for the PEP accelerator and a PEP detector. He was a charter member, along with Marty Breidenbach, Dave Hitlin, Harvey Lynch and David Leith, for SLD, the detector for SLC, the world’s first linear collider. Prescott proposed that the machine use polarized beams.

“The electromagnetic force is the largest force when you scatter off a proton, as in E-122, and the weak force is weak,” he said. “But at the Zpole (where Z particles are produced) in SLC, the weak force is by far the dominant force. The roles changed. Parity violation is a very large effect. Its full nature stands out.”

The polarized source for SLD took full advantage of advances in gallium arsenide made by Takashi Maruyama, Garwin and others, which together with a state-of-the-art laser enabled around 80 percent polarization of the electron beam.

The SLD collaboration published its complete and final results this summer together with the final results from similar experiments done at CERN in Switzerland. The polarized beam enabled SLD to make key measurements more precisely than any other experiment, even though SLD produced far fewer Z particles than the CERN experiments.

“At the Z pole, we had the best tools, we had the polarized electrons, we did the best measurement of the mixing parameter called the weak mixing angle,” Prescott said. The weak mixing angle is a ‘free’ parameter in the Standard Model whose value is not specified; it gives important information about the strength of the electroweak force and is a powerful tool for predicting the mass of the still sought after Higgs particle.

In parallel to SLD, Prescott returned to End Station A, collaborating with Ray Arnold and his group on learning the spin structure of protons and deuterons (a hydrogen with one proton plus one neutron) using polarized electron beams and polarized solid targets. They measured the spin of the quarks in the proton and the deuteron.

In 1986 Prescott became a full professor, and was associate director of the Research Division from 1986 to 1991.

“He’s very good, and he’s somebody I always trusted as a physicist and as an administrator,” said director emeritus Burton Richter (DO).

Current Research Continues

For the past three years, Prescott has been working on EXO to learn more about neutrino mass. Although EXO does not use an accelerator, the experiment still fits well with Prescott’s forte for probing the nuclei of atoms. EXO involves detecting certain rare decays from the nuclei of xenon atoms.

“The physics is interesting and worth doing and I like technically challenging R&D projects,” he said, neatly summarizing his four decades in physics.  Prescott now (shown above) and in 1975.

The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Tuesday November 22, 2005 by TIP